One conventional bidirectional optical communication systems is described in "Technical Digest Volume III of 5th International Conference on Integrated Optics and Optical Fiber Communication and 11th European Conference on Optical Communication" held on Oct. 1 to 4, 1985 in Venezia, Italy. The bidirectional optical communication system comprises a central transmitting and receiving apparatus and a plurality of subscriber's transmitting and receiving apparatus connected through optical fiber line thereto. The central transmitting and receiving apparatus comprises a plurality of transmitters for producing a plurality of transmitting signal lights f.sub.1, f.sub.2 - respectively, an optical combiner for combining the plurality of the transmitting signal lights f.sub.1, f.sub.2 - - - , an isolator through which the plurality of the transmitting signal lights f.sub.1, f.sub.2 - - - pass and in which power of the opposite direction thereto is absorbed, a turnable heterodyne receiver including an optical combiner for combining local oscillator light with signal light to be received from the optical fiber line, and a directional coupler through which the plurality of the optical transmitting signal lights f.sub.1, f.sub.2 - - - are fed into the optical fiber line and the heterodyne receiver receives the signal light from the optical fiber line. On the other hand, each of the subscriber's transmitting and receiving apparatus is structured in almost the same construction as the central transmitting and receiving apparatus described above except that a single transmitter for producing, for instance, a transmitting signal light f' which is fed through a directional coupler included therein into the optical fiber line is usually provided therein.
In operation, the transmitting signal lights f.sub.1, f.sub.2 - - - are downstreamed through the optical fiber line from the central transmitting and receiving apparatus while the transmitting signal light f' is upstreamed through the optical fiber line from the subscriber's transmitting and receiving apparatus. In the subscriber's transmitting and receiving apparatus, one of the transmitting signal lights f.sub.1, f.sub.2 - - - is selected by the tunable heterodyne receiver. The selected signal light is combined with the local oscillator light at the optical combiner included in the tunable heterodyne receiver. The light thus combined is output to an optical detector (photo-detector) to produce an intermediate frequency signal. The intermediate frequency signal is fed into a demodulating circuit to produce a demodulated signal after being amplified. The demodulated signal provides a subscriber with informations from the central transmitting and receiving apparatus.
In the conventional bidirectional optical communication system, however, the tunable heterodyne receiver is provided with the optical combiner in which the received signal light and the local oscillator light are combined to produce the combined light to be output to the optical detector thereby producing the intermediate frequency signal, in addition to the directional coupler through which one of the transmitting signal lights is received from the optical fiber line thereto and such optical parts as a local oscillator light source etc. For this reason, parts to be used in the apparatus inclucing a heterodyne or homodyne receiver are increased in number, and as a result there occurs a disadvantage that the manufacturing cost and volume thereof are increased.